Taking into account the height and weight of these antennas, their mast must be capable of withstanding considerable strain. Very strong girders therefore become a necessity.
To this end, instead of a mast of the standard type, embodied by a sole vertical member, which for the sake of strength must have an excessively large cross section and weight, masts having latticework girders are generally used, and these masts are therefore commonly known as "lattice masts".
These lattice masts comprise at least three vertical members, which are generally cylindrical and which are held together by a triangulation embodied by wind bracing devices, each of which is associated at its ends with two of the vertical members at points which embody assembly junctions.
To facilitate transporting them, it is known for these lattice masts to be embodied in a plurality of sections.
At the site where the mast will be used, the sections are then laid on the ground so they can be assembled end to end. Then the assembled mast is raised vertically, and it is kept in this position by a bracing system.
Each of these mast sections comprises three assembly parts joined by wind bracing devices corresponding to this part of the mast, which thus becomes a unit that can no longer be dismantled.
When they are used, these sections are simply placed end to end and assembled by some known means, such as a bolted connection, at each end to end junction.
Unfortunately, aside from the fact that a special machine, such as a crane, must be used to raise them, such masts have the disadvantage of being bulky, because each section is in the form of a latticework iirder having a cross section of considerable bulk.
In the case of the standard mast, comprising a single vertical member, the problem of great length is comparable and is likewise solved by embodying the member in a plurality of sections assembled end to end upon being used, and the problem of large cross-sectional bulk is also present, although less acute. Nevertheless, it is not at all necessary to use a machine to raise the mast, because for this kind of standard mast it is sufficient to mount the antenna on the top element, and then to insert this top element into a vertical guide shaft which is placed a certain distance above the ground, via a supporting framework; then, in order to raise the antenna to the desired height, the additional elements successively engage the bottom of the shaft, which progressively raises the top of the mast.
Unfortunately, with the lattice masts known at present, even if they are divided into a plurality of sections it is impossible to use this method to erect them, because the means for joining the sections and their wind bracing devices prevents their translation in a shaft; furthermore, taking into account the cross section of the lattice mast, this shaft would be of excessively large dimensions.